1. Field of the Invention
The present invention relates to a variable optical attenuator for use in optical communication and to an optical signal source for wavelength-multiplexed optical transmission systems.
2. Description of the Related Art
Conventionally, there has been known a variable optical attenuator having a structure in which the distance between ferrules opposed each other is changed to thereby vary optical coupling efficiency between the ferrules (see Japanese Unexamined Patent Application No. 8-94843). Also known are a variable optical attenuator having a structure utilizing the dependency of the intensity of a damped wave at the interface between different types of optical transmission mediums on the light incidence angle (Japanese Unexamined Patent Application No. 57-122403), that having a structure in which a disk-shaped optical filter having an attenuation factor varying in a sector manner is inserted into an optical path (Japanese Unexamined Patent Application No. 3-257414) and that having a structure in which two optical fibers arranged in parallel are optically coupled through a reflection mirror and an attenuation plate is inserted between the optical fiber and the reflection mirror (U.S. Pat. No. 5,226,104).
Conventional variable optical attenuators are provided with mechanisms for adjusting optical attenuation at positions perpendicular to the optical axes of optical fibers, respectively. If a plurality of variable optical attenuators are mounted on, for example, the panel of an apparatus, it is necessary to provide clearances between adjacent variable optical attenuators for operating the adjustment mechanism. This makes it difficult to realize high-density mounting. For the same reason, once the variable optical attenuators are mounted on the panel of the apparatus, it is impossible to adjust optical attenuation from the outside of the panel. Due to this, maintenance becomes difficult.
In case of a wavelength-multiplexed optical transmission system for transmitting a light beam after composing a plurality of signal lights having different wavelengths, in particular, there is only a narrow space in the panel. For that reason, it is required to easily mount and adjust the variable optical attenuators. It is also required to accurately keep output levels of the light beams of different wavelengths coincide with one another.